A simplified diagram of the type of system to which the invention relates is shown in FIG. 1. The system uses the principle of electromagnetic communication in which an interrogator containing a transmitter generates an electromagnetic signal which is transmitted via an interrogator antenna to an electronic label containing a label receiving antenna. The label antenna may receive a proportion of the transmitted energy, and through a rectifier may generate a DC power supply which may be used for operation of a reply generation circuit, connected either to the label receiving antenna or to a separate label reply antenna, with the result that an information bearing electromagnetic reply signal is transmitted from the label back to the receiver of the interrogator.
In the diagram of FIG. 1 the antennae within the interrogator and the label take the form of inductive loops. When separation between the interrogator and the label is much less than a wavelength at the operating frequency, the principal form of direct coupling between such inductive loops is provided by reactive energy storage fields surrounding those antennae and is expressed in terms of mutual inductance depicted as M1 in FIG. 1, between those loops.
In practical installations of electronic labelling systems it is desirable that labels be robust, easy to install and of low cost, All of these considerations suggest that the labels, and hence the coupling loop within the labels, be made physically small. Because some components within the label may also be fragile, it is sometimes desirable that the label be embedded well within rather than attached to the object being identified.
All these considerations combine to make the interrogation range which may be achieved with such labels and within practical restraints smaller than is desirable. This limitation is particularly relevant where the object to be identified is large, and its placement in relation to the interrogator is uncertain. Although the problem of limited interrogation range of the label can be alleviated somewhat by insuring that it is placed close to the edge of the object to be identified, there frequently occur situations in which objects to be identified are stacked, so that other objects prevent ready access of an interrogator antenna to an edge of the object closest to the label position.
One solution to this problem could be the use of a large antenna placed externally to the label, and directly connected thereto. However, such a solution generally leads to labels becoming of an unacceptable cost and fragility, and being of unsuitable dimensions for attachment to a variety of objects.